Devices for testing the rate at which a pile settles in the ground, otherwise known as "creep," in response to an axial load are known in the art. There are two types of settlement readings. The first type is "adshear," or "adhesion shear," which is movement of the pipe relative to the soil which is in contact with the pile. When the soil is frozen, "adshear" is referred to as "adfreeze." The second type of settlement reading is "shear," which is movement of soil proximate the pile with respect to adjacent soil.
The purpose of testing the settlement rates of piles is to predict the estimated creep that can be expected over a 50-year period, for instance. An acceptable creep over a 50-year period may be 1/2 inch. The accuracy of the equipment used in testing the creep of a pile determines the required duration of a particular test in order to determine the expected creep rate. A 0.5-inch creep in 50 years corresponds to 0.01 inch in one year and 0.00019 inch in one week. Conventional equipment is theoretically capable of measuring 0.0001 inch of movement of a test pile. Thus, in order to obtain reliable results, it is often necessary to conduct a test over a period of weeks or even months in order to maximize the amount of movement. Moreover, temperature and pressure variations of the soil affect the accuracy of the test readings.
The conventional pile testing device includes a test pile which is either driven into the ground or is placed in an oversized hole and backfilled with either a soil/water slurry or a sand, cement grout. A pair of horizontally disposed reference beams are disposed on opposite sides of the test pile above the ground. The reference beams are supported at respective opposite ends by reference piles which are driven into the ground at a distance of not less than 8 feet from the test pile. In this manner, the creep movement of the test pile theoretically does not cause the reference piles and beams to move. Measures are taken in an attempt to insulate the reference beams from sun and wind exposure to further minimize movement of the reference beams. Measuring instrumentation is connected between each of the reference beams and the test pile to calculate the movement of the test pile. As noted above, the measuring equipment is capable of measuring 0.0001 inch. Finally, a load is applied on the top of the test pile, forcing the test pile into the ground such that the movement of the pile can be calculated over time, providing a predicted creep rate.
As can be understood from the above description of the conventional device, the problems associated with such a device are that the accuracy of the test procedure is poor and the test is relatively expensive to conduct. Most importantly, it is difficult to ensure that the reference beams will not be affected by, for instance, the movement of the test pile and environmental factors, such as air temperature and wind speed, as discussed above. Accordingly, since the measuring accuracy of the conventional test procedure is poor, the duration of the test must be relatively long in order to maximize the movement of the test pile to within a range that can be reasonably calculated to any degree of certainty. As noted above, tests lasting as long as four months are not uncommon. To accurately determine creep rates, a displacement measuring system capable of measuring microinches per hour is required